Treatment of echoes in transmission systems



Aug. 8, 1933.

H. A. AFFEL 1,921,063

TREATMENT OF ECHOES IN TRANSMISSION SYSTEMS /m/erfer BY mn/jf@ ATTORNEY mit :American Telephone and Telegraph'o'mponyi;

VAend ,.mvore -perticularly to signaling fsystems v -Which echoes :of ythe orgi duced.. l v, 5 @ne .ot vfthe, 'objects of, the inventionV is the elimination or reduoti n of' echofeife'ctset Vthe re.evi-11e-'end'of5e.ntransmissionSystemti .i

Another objectgof the inyentionlis thedeereese of' vintelligibility l of 1 the gsignelsyduring transmis- ,s1on. p y 1 1 `The Vf,ulrid'znrientel videe. involved in the .invention is thlhalancine out. of; theV ehes ,tierismitted tothe'receivng circuithyeehces darti-1 cilly produced iirthatfcircit. 'This 'Cancel- "ngeop'eration perni-its th :e nnetion of` eehoes A unevoiieb'ly transmitted; 1 also" permits theY ar'- ticiztl 1 introduction, tzttha. transmitting end the!systeinofjeehos l s eryeto decrease'` he liritellig'ilility tof thesignalsduri Tensmisson; since these distorting'eches crtvbefrenioveii'in the `lre'cei/lgCiCJlt. v Q Other olijects end featuresF "ntheunventiorr vwill 'ppear -from Athe A'fsolle-Win'@5discus's'ionfamd from thefollowing `detauiled 'descriptioniof 1specic applications :of: the #intenti e Aidees.

Y -flhe .desorintivmatter a.p'peeringjshereinafterI dra-Wing indicates thezgenereftemywhich. generation maybe artiiicialondjfor the purinose off introducing secrecy, and shows et the Avreceiving end of -the;.s'ysten1.an orratnge` nient for balancing out f the f transmitted echoes ,u fhentloeecho energyiisfattenu ltothe originelenergy. i m 'q- @modified @fr thonf the original energy.

secrecy, in :,Whieh distortion is introduced at the mission of 4signals. s Figi ldCBS 'the A fcantsffinementl ide'to themeesureinentin 'the reoeivingoircut-oi echoes transmitted over "the system: A; "ikej hara-.eters of-feferecesinwetgever-slingnal jwaves, ere Vintro p f etedwithfreeton.

angeirient iolxoff" setting the `echo efie'ctsin-.arceiving-fcircuit when the echo eneng'yisV of `Ylie/neigmagnitude y s 4 1 foyer aflinefL to the'distent-receivingffcircuit RC g ures 'of the 'dr ments;

The origin ofV4 echoes `in accoustical and elec-. trioel transmission .systems and 'the'.l desirability :of eliminating the eiiects-of=,echoes in simlrsys-VV tems/ere .Well understood in the ort. Jn; en l `audil ewing'-designate@correspondinglejtoriumffor instance, the voice 'offa speekermay loe heard over the-'direeteirpethto-the listener. Y

and elsoaover indirect paths involving Atherefie :tionfVoi-thesound Weves'from the wells -orthe peilingr .of {th'e auditorium; A Again, '-speeh. er music" may loe-received from ftwo `orrno're radio ytroL-nsmitters simultaneously, vand thetransinith'transniisson circuits having diierent electrical distances. )Furthermore the introductie' 1- an? preciable-echoesfin long two-Wire ,and'Lfor-Wire `telephone crc'uitshas"beenfoisolne-timee. prob- 1em l selling 'for treatment and solution 'fby telej'phoneengin'eers.

y l-I- ifhefv appreciable result y of lan echo Vistlfiotrithe u originalfsound is repeated with o. time delay y'of i rconsidereble" length. A:The 'seriousness offtheecho i' ylires are 'employedethentoehminate'the source' Y "of the eehoe's or tofredime the delay inter-vel or: Y.

I the Jmagnitulij of'fthe reflectedjener'gy;hr1* both, 85 l esV far as'iis possible;

"lthe present Li'm/entilon iSFtog-belenceout flthe `"transmitted fechoesl in 5 the receiving l`eirfcuitV by locally produced echoes "of A'like ehare,jeteristies.` vWitn reference 'first to lFig. of the -drafwingfiet Karo -nsorne yinelnner introduced; "The-jenergy" enshrined 4from 1the. einsamen-ng fen-'Cuit rrc Vfthe" expiration Of the 'i's't'ulit"fv'timegithe eliit beessumed -thetfaltthe:transmitting endiTC represent only the original energy free that is, during thisrst time interval Vthere is *impressed on the receiving circuit only the Ycom- RC will of echo;

ergy receivedv in ythe receiving circuit ponent a., In the, second time interval there -Wil1 arrivethe second. component-hand along:

with itthere will arrive the echo of Vou `multiplied byfsome factor' zc, which in the case assumed is a fraction.

`.val the component c will have added to it arb', which Vis the echo ofthe original component ZJ,

snnnariy, during @heten-d time interfeto.' Thuawthe received'energymay be represented according totime intervals by the follow- In the arrangement OffFi-. ill the .applicant passes the Venergy transmitted overY line L `.through a unilateral vacuum tubel circuitVTi to a transformer TR1. An youtput path leadson to .a receiver R and-abranch lcircuit is .provided for feedingA energy back to the transformer TRiatY the input of the receiving circuit.

` naflsof thefinputfwndmgarendemgnatedf'as 17cm-ding tov time vintervals equal to the delay The v termi- ,and lathe-terminals of the outputV winding as 3 and 4 yand the terminalsY of the feedback winding as and 6.' In this feed-backA circuit are includedan attenuator'rrln,ya,V delay circuitl D1,

. [7. and agsecond unilateral Vacuum vtube circuit VTz. f Y The potentialapplied -to the terminals l- -,2rfoi lows the tablev given above. Thus, during the rst time'interval the wave a is received .and is..

passed lon tothe receiver `R unaffected/by the back lenergy isropposeduto the :incoming energy,

lasindicated in Fig.` 1 the ,transinittedrecho mar' feed-back arrangement. ifi-part' of lthis* energy, however, is diverted at point *I 8 and is returned vto the inputcircuit through winding 5'6, is attenuated by :c in A1, and is subjected to adelayl in D1 which is equal `tov the ldelay of the transfed back-to the terminals 5;,-6 of the transform,u er; it will he` understood,further-thatiffthis Yfed andthe locally producedI eohofra oppose each fother, and since the potentials areofjequalV mag-- ntude the two echoes are balanced'out andthe 'resultant wave whichjpass'es on to the receiver Rist, free of the echo ma.; .i l v I f- Similarly,a party: of b is /divertedat the point ,7-8, is attenuatedY and, delayed in the feed,-4

back circuit, and serves to balance out the echo afb which accompaniesthe wavev c corresponding to thethird time interval, etc. It is-thus seen f lthat while vthe energy ,which is. transmitted and receivedrepresents the signalgaco'ompanedwby asubstantial echo, thefenergy iinally to thereceiver Rl is ,free of echo.

. y:it wiuzbe understood that the'un'iiaterai dpassedk on jvicefVTl prevents the; echo balancing'vcurrents from-,returning to the line L,-'and that-the unilateral deviceV AVTQYserves to .permit the balancing currents to pass .in only one direction:

- It is evident to those skilled inthe artythat unlessthe feed-back circuit ofy Fig. l Ainvolves "anet loss, there willbe altendency toward sing- Sing in the receiving circuit RC. -Thus, Yit is seen dicatedv in Fig. 2.

ynals 3--4 of a transformer TR2.

the receiver` ,Rr

layed by onetime unit.

Timeintervan 1 2 f 3 4 5 .Composite wave .r-l7- ll+a transformer a wave which'may be representedl as follows:

that the arrangement of Fig. 1 will function well if the echo attenuation factor x represents a considerable difference of magnitude between the original energy and the reflected energy. Since, however, there are echoes which exceed the original energy in magnitude, it becomes desirable to provide a modification of the arrangement of Fig. l1, which is not subject to the hazard of singing. Such an arrangement is inv Y V in the case of Fig.`2',the energy coming in over lineL from a distant-transmitting point is passed` through a unilateral vacuum tube cir- I cuit VT1 and'is causedto Adivide at point 1 2.

A portion of thel energy passes over the upper `loranchthrough a delay circuit D1 tothe termi- The other portion of the'energy passes over rthe lowerY x branch and through an attenuator A; and a unilateral device VTZ to the terminals 5-6 of 95 theV transformer TR2'. lThese two portions of the incoming energy arev then combined so as to oppose each other and to produce a resultant potential across the winding '7 8 of the transformerTRz'.v This joutput then passed on to :100

V The rincoming "energy which is divided actime, maybe represented by the following table:

'rimeinternal 1 v2 3V 4 v5 fnalsg3`4 ofthe transformer TR2 may be repre# sented as follows: l Y J Compositewave 0 Y a nlother words, the; transmitted waveV is de- The energy which is passed ,through the'attenuator A1 will appear at terminals 5f6 of the transformer asfollows:

C I 6 y y Y W @+51 When'these two jbranchp'ath outputs are combined inv opposition in the transformer TR2,

therer results across the terminals ,7e-8 of the 'rimeinrerval 1 `2 a 4 5 Compositewvave v.--'.-- 11.* y .Ilv

Il y

` able .v improvement introduced'with a largereduction of the echo effect. It will, beseen'from vthe last tabular picture that there is some distortion of theV originalpsignal but that the disltortingcomponents are. greatly attenuated.y This 150 Licences .jY

. is, of. -cours`e1, 4a favorable-.condition :ascompared v therein:disclosed-#and also the arrangement disaclosedxin Fig. 2V- serves to improve therese@ tion in the'V receiving: circuit RC of signals."

transmitted from:the'. distant transmitting cir-j- '.cuitTG.' v. In vthe' normalY case, of course, the

echoes generatedinfTC areunavoidable and the 4only thought .isxtofeliminate or reducer the ob-V -jectionable effect at 'the receiving end of :the system. However; the applicants 'balancinglarl rangement .at :thereceiving end permits theiarticiaflrdistortion of Athe transmitted energy yatr the transmitting end, for.y the V'purpose of reducing the intelligibility `of the signals during transmis-' sion with a viewltofsecrecy. In'theV arrangement of'xFig. 1, the energy traveling overthe -line Lis divided at Athe 'point 9-10. The por@v tion .of the yenergy passing over .the upper branch "reaches the-point 1'1- 12 Witlfioutvdistcrirtion. .portionwhichpasses through the lower f ibranchghoewevenis subjectedA to, a `delay 'in the delayV circuit VD and preferably to an attenuation =inf the vati'enuator A. i Accordingly, Wherithis portion of the energy `is 'recombinedwith the` other at points`11-12the energy sent 'o'n'over the line L 'has the `form ofthe originalfsignalv accompanied'by anA echo. Itvwill b e understood thatthe unilateral vacuumtubecirCuitsVTv and VTffare included to insure `the unilateral action :of the'bra-nch paths andmayintroduce-.gain or loss if desired for' other:purposes.A When-the distorted energy reaches fthedistant receiving circuit it is, of course,` 'desirable -torestorel the 'normal intelligibility -ofthe vsignal,` and; the

above-described technique is employed" whereby echoes are produced in the receiving circuit and are opposed to the incoming `echoes transmitted from the-transmitting end. 51"*"5 VThe idea just A"discussedcan, ofcourse, be carl fied to a further point asxind'icatedl in etheiarrangements of Figs; Sand :4': 'Inflig 3 at the signalj apppear-as low frequencies,fand'thelow 'only the distortion of the artificially introduced echobut al'sothe `distortion of'irequency -inver- 'j sion. Y"lin theloWer-brancwpath of the* circuit.

TC, along with the delay circuit ,D-and theat- `li-kecircui-t -inlwhich the high frequencies of the frequnciesfas high. For instance,fthe inver# sion may be arranged to takeplace rover a: fre? 'quencyfrangeof about SOOQ cycles :fa-speech frevr'quenc'y of 300 cycles, Afor instance, :will appear in theutputof'the inverter as 2700 cycles and,

similarly,- a'hig'li speech frequency of 2500 cycles, for instance, will appear in the output-as 500 cycles. At the'receivingfcircuit RC thefeedibackarrangenient of'Fig l islelnployed, Vandto thfe"attenuator."Al and the idelay circuit D1 -is added an' inverter Tf1.l l The Vdistorted Wave prof 'duced at'the terminalsr-.G 'of the' transformer TR1 is opposed toftlfiedistorted' Wave transmitted for'the purpose oifsecrecyfl and the two are canceled out,"lean'iirgilthe,signalI oifnormal iiitelligibility to bepas's'edon to'therec'eiver R..

' lm the arrangementof Fig. 4, `thejenergy in` chicles merely `the: iunilefter.al device K7T.: The

vthirdgbranch 13a-th' :includes: a i unilateral deviceA VTQ: a'fdelay circuit -D and anfattenuater A2.

yIt -Will beunderstood that -the dela-ys vintroduced t by Dgand D are -ldiffferent and that .the :attenua-v tions fintroducedby Vligand 1A are different. At thereceiving end itwo ieed-.backicircuits are .provided, vthe :firstincluding :anat-tenuator A1 .and a V.Ide-lay :circuit Ligand the second including-an .in 1t-his gattenuatorAZ-fand a' delay circuit D2. receiving circuit :RC .there are-diiiierences .erfde- )lay-.and :attenuation :corresponding .t-o tlriosev v-in- :troduced rat` the transmittingffend, A`and raccord.-

sof

'ing1y,- VWaves resulting :at the-terminals. --fand ll-gotfthe.transformer TR1 serve to remove ,the distortion'y introducedl at the .transmitting fendi, l Y 1.- 1 Wit-h reference toFigyi, it Wil-l ibeunderstood thatthe inversion` ideaofl Fig. 3 may .be com bined with the multipleech'o ideaof Fig.- 4. If

this `is desired, an inverter "I1 is included one :of fthe Vbranch pat-hs, for instance., lof the TC, andV correspondingly, an inverter Iris-1 ineluded in one circuit RC;

shift of, theV echo path waryfwiththc frequency; Such variations, if known, can :be imitatedyto make thereliinination ofi-the. echoes complete. "Thusfit'willbe Iunderstood that vin the 'arranger `mentso'f ;Figs and 4 .forms ofi'distortionotherf than frequency .inversion might `Well beused. f

V"Iherorigi-nai Vener-'gycnrfiponents of the received wav'eare passed'on `:to `a' receiver, While iajpor- 1GO f j the .freed-back circuits ofthe In generali/the transmission loss Aand the .phase ,tot

iro;

tion yof each component "is diverted at the point '7--8 and after',l being subjected to atten-nation' fandndelay is. red- ?.ioaeif'to ine-minnaar lthe circuit 'inoppositioii rto thejincoin-in'gI echo energy. transmitting'circuit TC the-reds introduced not V p 'urpose measuring echoes, the attentif *l and the delay'circuitjoffthe.feed-.back cir- Vcuit 'fsli'ouldiV be adjustable. The operater'ivill listen at the receiver and will adjust the'attenu- :ator andv the'delaycircuit until therec'eption is wisedeterminethe degree of attenuation introyg ducediingthef f eedfiback circuit andthe degree 'of delay: i in `fthe delayl circuit therein. Sincei the "absen'c'eof echo.' meansthat there isfa -balancein thejlfinpuft ofthe circuit, thesey determined arnollints of attenuation and 1 delay' uiilepresent -While the. invention has beenY disclosed in. cer- "-t je magnitude fandthe delay' "offthev received ltainembodiments for the,purposefof illustration, i

echoes which consists, in producinglinzthe re-f yceiving circuitechoes havingthe same characfxteristics asth'ose .previously` introduced, .and opposing Vtoeach .otherin said circuit the trans- .mitted'and'the locally produced.echoes. I

' 2'. In asystemv in" which waves representing Asound are-transmitted to a receiving circuit and in ywhich waves which are echoes of the original waves, delayed with'relation thereto, are introduced vin the course of the transmission, the

method of'eliminating the echo waves in the receiving circuit. which consists in producing therein-echo waves-like thosel produced in the lcourse of the transmissiomand opposingto each other in the receiving circuit the transmitted and the lccallyfproduced echo Waves.

I3. In a system'in'which waves" are transmitted to a'receiving circuit and in whichechoes of said f vwaves are introduced, the kmethod ofimprovingr inY which echoesz'are introduced'having the same lfo'rms'fas the original waves Abut delayed with relation thereto and having avv difference ofvxnagnitudetherefrom, the method of improving the reception ofthe sound `atthe receiving yend oi the system which consistsin dividing the incoming energy into portions, delaying the transmission of onepoitionby a time equal tothe delay' v of the transmitted v echoes, attenuating one por- .tion by 2 they dierence. lin2magnitude` between the v .original and the-echoenergy, and recombining Vthe'portions. of vthe received energy in opposition.

`5.In Vva system in which waves representingA sound are .transmitted to a receiving circuit and in which echoes areintroduced having thesame .form'sY asA thek 'originallwaves but;l delayed withrv time rinterval separating the original waves and relation thereto by Ya certain time interval and having adifference of magnitude therefrom, the

method of improving thereception of the vsound at vthe receiving endY of the. system which` cony sists in. dividing the transmitted; energy.y .between Vbranch paths, introducing in one lof the branch pathsanfartiiicialV delayfequal'tofthe certain time.intervaLintroducing in`V one of Vthe branch ,paths anv artificial change. of frna'gr'iitude`, and 'combining the outputs of *thebranch paths in opposition.vr

.6.4 Ina system in' which jwaves are transmitted i, 'toa-receiving vcircuit vand in whichy echoes of f such waves arev introducedlmeansin the receivingcircuit for producing echoes-resembling those transmitted, and meansfin saidjcircuit for opposr[fing to each .other the transmitted andthe locally y produced echoes.

'7.,1In auhsystem `inlwhich waves-.representing sound are transmitted to` areceiving. Circuit and in which echoes of such Waves areintroduced 8 In a vsystem in whichwaves'vv representing I soundr are transmitted: to a receiving circuitl and 7c infwhich there are introduced ec'hcjwavesv hav.-

' ing a certain attenuation andY afce'rtain delay with'relation to the'original waves, the Vmethod ofY eliminating the echo effects in thevrec'eiving circuit whichconsists),` in diverting a portion 'of vthe-orginalenergy Whichhas passedthe input r1,921,0fre, Y

of saidcircuit, subjecting the diverted energy to attenuation -and delayequal to the attenuation and delay-of the transmitted echo, and feedingthe attenuated and delayed divertedenergy back to saidrinput Yso as to balance out the incoming echo energy.l

9. In a system in which waves .representing sound are transmitted. to a receiving circuit and in which there are introduced echowaves havingVV a. certain. attenuation and a certain delay with relationto'the originalwaves, meansin the VVoutput oi the receiving circuit for diverting a portion of the energypass'edfthereto, means for subjecting the .diverted energy to attenuation and delay equal to the attenuation and` delay of the ltransmitted echo,A and means .for feeding vthe vattenuated and delayed diverted energy Vback ,to the .input Aof the receiving-circuit in oppositionto the incoming echoY energy.

i 10.Y In a system in which waves representing sound yare transmitted to a receiving-circuit and in which echoes of said waves are introduced, the vmethod of reducing the echo effects vin. the receiving circuit which consists in divid` ing. the energy entering. the receiving. circuit into twoportions, .subjecting one portion to a delay, -subjecting'the other portion toan attenuation, .and combining the two portions vof the energy-at theoutput ofthe receiving circuitin opposition. .Y 11. In a system in which Ywaves representing sound aretransmitted to. a receiving circuit'l and in which there are introduced echo waves like lthe original waveslin form but differing therefrom inV `magnitude andy .delayed with .relation thereto yby a certain.v time intervaLthe method srs otreducing theechoeects inthe receiving cirvcuit which vconsists in dividing the energy en-f transmitted echoes, subjecting .the other portion `ftol an attenuation equal tothe difference of magnitudegfbetween the original vwavesiand lthe transmitted echoes, and combining the two portions ofthe energy atthe output 'of .the receiving circ/uit in opposition, substantially as -d e scribed." i. Y i

`12..;fn a systeinin whichiv/aves representing Ysound are transmitted toa receiving circuit and in which there aleintroducedecho'waves, means in the receivingl circuitr vrorhdividing the incoming energy-intovtwo portions, means 4for delaying `the .transmission of onejofv saidrportions,

ineanshforl attenuating the other of said portions-,1 and means for combining the two portions in opposition thereafter.

`1.3.V ln a signaling systemVthefmethod delike those. produced at the transmitting end,. and

balancingioutfatthereceiving en d the transmitted and the.. locally Produced echoes.

14j. `n a '.'system for theftransmission of signal waves, fmeans at the transmitting end of the system for introducing echoes 'of the transmitted waves,Y means at the receiving end oi" the system' for producing echoes ofthe transmitted waves, and meansat the receiving end .ofthe system for canceling out the transmitted and transmission and y is irestored (to Y .t t Y 1.1971215063 i the locally produced'echoes; wherebyV the intelo ligibilty cf thesignal Waves is' decreased'during normal at .the receiving end of thesystem. i

151.` 'In asystem for the transmission-of signal A'Wa'ves, meansat the transmitting end of the vsys-- tem for introducing echoes of thetransmitted.` Waves, means atthe receiving end ofthe systemV for producingrechoes of the Vtransmitted waves f equal in, delay andzattenuation'to the echoesintroduce'd at the transmitting end,` and means at -the receiving end of the-system for'canceling 16. In'a ,signaling system,Y the -metliodof de- Y creasingtheintelligibility of Vthe Ysignals during transmission and restoring the Anormal intein out the transmitted fand'r the "locallyv produced gibility at the receiving end ofthe-system, which consists in producing echoe'sof .the signal Waves -at the transmitting end, inverting the `frequencyfband` of the echoes so produced'transmitting to the receiving end both' theY signal Waves and the distorted echoes, vproducing'at thereceiving sof A '17.- In asystemior the transmission of signal Y, v- Waves, .means at the transmitting end for prov ducing echoes' of thejsignal Waves, meansfor` inverting A thel frequency band of the echoes, j A likev those produced at the transmittingfend,V .y

means at thereceivng end for producing echoes meansfor inverting the frequencyband of the; echoes produced at the receiving end, and means at the Yreceiving end for balancing out the transl initted and "the l locally produced distorted echoes. v i Y 1'8. Ina system in which WavesI representing sound aretransmitted to arreceiving'circuit and in which echoes. are introduced, the method of measuring the echoes in thevreceving circuit which consistsin diverting a portion of the orig` inal energywhich has [passed the input lof said;`

ff'circuit, subjectingy the diverted energyto attenuation and delay, feeding the diverted energy back tosaid' input so as to oppose the incoming V95 echoenergy, observingthe sound output of saidl circuit, adjustingthe attenuation and the delayY -to Which the diverted energy isv subjected until said outputis free ofappreciable echo, andde-r` terminingthedegreerof attenuation and delay i0()z the echo-freefresult.

introduced to produce v 3 *HERMAN A. AFFEL. Y

Y Ik 1117.01

'Vf-12ov Y' rsa1 

